Determining the Larmor Frequency for NMR Tools

ABSTRACT

The Larmor frequency for an in situ nuclear magnetic resonance (NMR) tool is determined and used to acquire NMR data. An NMR tool is provided and placed in situ, for example, in a wellbore. An initial estimate of the Larmor frequency for the in situ NMR tool is made and NMR data are acquired using the in situ NMR tool. A spectral analysis is performed on the NMR data, or optionally, the NMR data are digitized and a discrete Fourier transform (DFT) is performed on the digitized NMR data. The modal frequency of the spectral analysis or DFT is determined, and the Larmor frequency for the in situ NMR tool is determined using the modal frequency. The NMR tool is modified to transmit at the determined Larmor frequency and then used to acquire further NMR data.

CROSS-REFERENCE RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/728,961 (20.3223-US-NP), filed Mar. 22, 2010, which isherein incorporated by reference.

BACKGROUND OF THE DISCLOSURE

Nuclear Magnetic Resonance (NMR) tools used for well-logging or downholefluid characterization measure the response of nuclear spins information fluids to applied magnetic fields. Downhole NMR toolstypically have a permanent magnet that produces a static magnetic fieldat a desired test location (e.g., where the fluid is located). Thestatic magnetic field produces a non-equilibrium magnetization in thefluid. The magnetization is aligned along the direction of the staticfield. The magnitude of the induced magnetization is proportional to themagnitude of the static field. The proportionality constant is thestatic magnetic susceptibility. A transmitter antenna produces atime-dependent radio frequency magnetic field that is perpendicular tothe direction of the static field. The NMR resonance condition issatisfied when the radio frequency is equal to the Larmor frequency,which is proportional to the magnitude of the static magnetic field. Theradio frequency magnetic field produces a torque on the magnetizationvector that causes it to rotate about the axis of the applied radiofrequency field. The rotation results in the magnetization vectordeveloping a component perpendicular to the direction of the staticmagnetic field. This causes the magnetization vector to precess aroundthe static field at the Larmor frequency. At resonance between theLarmor and transmitter frequencies, the magnetization is tipped to thetransverse plane (i.e., a plane normal to static magnetic field vector).A series of radio frequency pulses are applied to generate spin echoesthat are measured with the antenna.

The resonance condition requires that the transmitter radio frequencyequal the Larmor frequency. Deviation between the two frequencies canlead to inaccurate estimation of porosity, in the case of logging tools,and hydrogen index, in the case of fluid sampling tools. In addition,the deviation can lead to systematic errors in the estimation ofrelaxation time distributions, thereby resulting in inaccurate estimatesof, for example, viscosity, permeability, pore size distribution,irreducible water saturation, etc.

The Larmor frequency when operating in downhole conditions differs fromthat in the laboratory. This difference is caused by the temperaturevariation of the magnetization, and in some cases, by the accumulationof magnetic debris in the vicinity of the permanent magnet. Magneticdebris is frequently found in the drilling mud due to the drill stringscraping metal particles from well casing while tripping in and out ofthe hole during drilling operations. The effect of magnetic debris onthe Larmor frequency depends on the quantity and distribution of thedebris. As a result, the Larmor frequency needs to be determinedaccurately for downhole conditions. Two methods are currently known forin-situ estimation of the Larmor frequency.

One method, the Larmor Frequency Search Task method, was developed byFreedman, et al and is described in U.S. Pat. No. 5,457,873. In thismethod, an initial estimate of the Larmor frequency is made based on thetemperature of the tool. A series of NMR measurements are made atdifferent operating frequencies. A predetermined response curve isfitted to the measurements to determine the frequency at which themaximum echo amplitude is obtained. The implementation of the method isillustrated in FIG. 1. The figure shows a plot of the initial echoamplitude calculated from the mean of the first ten echoes for a rangeof transmitter frequencies. The maximum amplitude is observed at 2.270MHz, which corresponds to an independent measurement of the Larmorfrequency using a Hall probe. However, the implementation of thisprocess is time consuming. Additionally, the process requires that theformation porosity remain constant during the measurements, which maynot be true if measurements are performed while the tool is moving.Moreover, in low porosity formations, it is not possible to accuratelydetermine the Larmor frequency because of low signal-to-noise ratio(SNR).

A second method, the Echo Phase method, was developed by Bordon, et aland is described in U.S. Pat. No. 7,026,814. If the Larmor andtransmitter frequencies are different, the phase of the echo signalchanges along the echo interval with respect to the reference radiofrequency phase. The difference between the instantaneous phases of theecho signal at two time intervals is linearly related to the deviationbetween the Larmor and transmitter frequencies, provided the deviationis small. However, the implementation of this method requires a detailedcalibration of the electronics (e.g., the phase shifts due totemperature need to be recorded). In addition, the phase difference isinfluenced to a large extent by antenna tuning (deviation of theresonance frequency of the antenna from the transmitter frequency).Therefore, the detuning of the antenna and its effect on the phasedifference needs to be calibrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a plot of echo amplitude vs. transmitter frequency that may beused by a prior art method to determine the Larmor frequency. Themaximum echo amplitude is obtained when the transmitter frequency isequal to the Larmor frequency.

FIG. 2 is a plot of echo amplitude vs. time of an echo signal obtainedusing a Larmor frequency of 2.270 MHz, according to one or more aspectsof the present disclosure. The signal is digitized at a samplingfrequency of 50 MHz for a record length of 1 ms.

FIG. 3 is a plot of the normalized magnitude of the discrete Fouriertransform of the digitized echo signal of FIG. 2, according to one ormore aspects of the present disclosure. The x-axis is scaled to thefrequency range of 2.2 to 2.3 MHz. The modal frequency corresponds tothe Larmor frequency of 2.270 MHz.

FIG. 4 is a plot of echo amplitude vs. time of an echo signal obtainedusing a Larmor frequency of 2.270 MHz, according to one or more aspectsof the present disclosure. The signal is digitized at the samplingfrequency of 10MHz for a record length of 50 μs.

FIG. 5 is a plot of the normalized magnitude of the discrete Fouriertransform of the digitized echo signal of FIG. 4, according to one ormore aspects of the present disclosure. The x-axis is scaled to thefrequency range of 2.1 to 2.4 MHz. An insufficient resolution of 20 KHzis obtained in the frequency space.

FIG. 6 is a plot of the normalized magnitude of the discrete Fouriertransform of the zero padded echo data of FIG. 4, according to one ormore aspects of the present disclosure. For reference, the normalizedmagnitude of the DFT of the unpadded data is also shown.

FIG. 7 is a plot of the real and imaginary components of the Fouriertransform of the zero padded echo data shown in FIG. 4, according to oneor more aspects of the present disclosure.

FIG. 8 is a flow-chart diagram of at least a portion of a method,according to one or more aspects of the present disclosure.

FIG. 9 is a schematic drawing showing an NMR system disposed in awellbore, according to one or more aspects of the present disclosure.

FIG. 10 illustrates a wellsite system in which a while-drillingembodiment of the present invention may be used, according to one ormore aspects of the present disclosure.

FIG. 11 is a schematic drawing showing an example while-drillingsampling tool disposed in a wellbore, according to one or more aspectsof the present disclosure.

FIG. 12 is a schematic drawing showing an example wireline tool disposedin a wellbore, according to one or more aspects of the presentdisclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

The Larmor frequency of an NMR tool can be determined in situ byperforming a (frequency) spectral analysis of the spin-echo or freeinduction decay signal. For example, the spectral analysis may be doneusing a Wavelet transform or from the discrete Fourier transform (DFT)of the digitized NMR signal. Other transforms or spectral analysistechniques could also be used.

An analog NMR signal is a superposition of sinusoids in a range offrequencies centered on the Larmor frequency. To digitally extract theecho data from the receiver, at least two paths may be taken. One methodinvolves direct digitization. If fast analog-to-digital (A/D) conversionis available, the echo signal can be directly digitized and recorded.FIG. 2 shows an example of an echo signal obtained using a Larmorfrequency of 2.270 MHz digitized at a 50 MHz sampling frequency.

The second path involves the conversion of the echo signal to anintermediate frequency. The echo signal is mixed, or multiplied, with areference signal of frequency, f_(r). Mixing the two signals creates twooutput signals at two different frequencies: one at the sum of the twomixed frequencies, and another at their difference. The higher frequencycomponent can be filtered, and the component at the differencefrequency, called the intermediate frequency, is digitized and recorded.This technique may be used if a fast A/D converter is not available.

Regardless of how the digitized signal is obtained, the Larmor frequencycan be estimated from the DFT of the digitized signal. To illustrate,let the digitized signal be denoted by x(n), n=1,2 . . . N, where N isthe total number of samples in the record. A DFT of the signal resolvesthe frequencies present in the signal into a discrete set using thefollowing transformation:

$\begin{matrix}{{{X(k)} = {\sum\limits_{n = 0}^{N - 1}{{x(n)}{\exp \left( {- \frac{{2\pi}\; {kn}}{N}} \right)}}}},{k = 0},{{1\ldots \mspace{14mu} N} - 1.}} & (1)\end{matrix}$

X(k) is a complex number whose magnitude represents the amplitude of asinusoid with frequency k/N cycles per second.

As mentioned earlier, an NMR echo signal is a superposition of sinusoidsin a range of frequencies centered on the Larmor frequency. This rangeof frequencies arises primarily due to the inhomogeneity of the staticmagnetic field. The frequency spectrum of the digitized echo signal canbe resolved using the Fourier transformation of Eq. (1). If a directdigitization scheme is used for data acquisition, the Larmor frequencycorresponds to the modal frequency, ƒ_(m), of the DFT magnitude. FIG. 3shows a plot of the magnitude of the DFT of the digitized echo signalshown in FIG. 2. The mode of the DFT magnitude corresponds to thecorrect Larmor frequency of 2.270 MHz.

On the other hand, if data are acquired using an intermediate frequencyconversion scheme, the Larmor frequency can be determined from the sumof the carrier and the modal frequency as shown below:

ƒ_(L)=ƒ_(r)+ƒ_(m)   (2)

In the above equation, ƒ_(L) and ƒ_(r) are the Larmor and referencecarrier frequencies, respectively.

For accurate and precise estimation of the Larmor frequency from the DFTof the echo signal, the NMR signal should be digitized at least twicethe expected Larmor frequency to avoiding aliasing effects. This is inaccordance with Shannon's sampling theorem which states that the maximumfrequency (Nyquist frequency) that can be accurately resolved is equalto half the sampling frequency. In addition, the resolution in frequencyspace is inversely proportional to the total record length, and is givenby:

$\begin{matrix}{{\Delta \; f} = \frac{1}{T}} & (3)\end{matrix}$

The requirement that the signal needs to be digitized at least twice theLarmor frequency implies that the total number of samples, N, in therecord length is:

$\begin{matrix}{{N = {\frac{f_{s}}{\Delta \; f} \geq \frac{2f_{L}}{\Delta \; f}}},} & (4)\end{matrix}$

where ƒ_(s) is the sampling frequency. A resolution on the order of fewKHz is typically required for NMR logging tools. Therefore, for a Larmorfrequency of 2 MHz and a resolution of 1 KHz, the number of samplingpoints is at least 4000, and the record length is at least 1 ms.However, due to memory limitations, the echo signal in NMR tools isgenerally sampled for a much shorter time. For example, FIG. 4 shows theecho signal at a Larmor frequency of 2.270 MHz digitized at a samplingfrequency of 10 MHz for a record length of 50 μs. The correspondingnormalized DFT is shown in FIG. 5. The resolution of the frequencyspectrum is:

$\begin{matrix}{{\Delta \; f} = {\frac{1}{50 \cdot 10^{- 6}} = {20\mspace{14mu} {KHz}}}} & (5)\end{matrix}$

Thus, the resolution in the frequency spectrum needs to be improved,without increasing the record length. This is accomplished by appendingthe digitized echo data with zeros at the end. This process is calledzero padding. The required resolution in the frequency space is obtainedby adding N_(z) zeros as shown below:

$\begin{matrix}{{\Delta \; f} = {\frac{f_{s}}{\left( {N + N_{z}} \right)}.}} & (6)\end{matrix}$

FIG. 5 shows the magnitude of the DFT of the echo signal in FIG. 4. Thedigitized data can be zero padded such that a frequency resolution of 1KHz is obtained. The number of added zeros, N_(z), is calculated fromEq. (6) as follows:

$\begin{matrix}{N_{z} = {\frac{f_{s}}{\Delta \; f} - N}} & (7)\end{matrix}$

For the particular case being considered,

$\begin{matrix}{\left. \Rightarrow N_{z} \right. = {{\frac{10 \cdot 10^{6}}{10^{3}} - {\left( {10 \cdot 10^{6}} \right)\left( {50 \cdot 10^{- 6}} \right)}} = 9500}} & (8)\end{matrix}$

The mode of the DFT is located at the correct Larmor frequency of 2.270MHz, as shown in FIG. 6.

It should be noted that, in addition to the magnitude of the DFT, thereal or imaginary components of the DFT of the zero padded data can alsobe used to estimate the Larmor frequency. For an echo signal with aphase angle of zero, the modal frequency of the real componentcorresponds to the Larmor frequency. Additionally, the imaginarycomponent of the DFT has a zero crossing close to the Larmor frequency.Those two results are, however, valid only when the phase of the echosignal is zero. Therefore, for the real or imaginary components of theDFT to be used to determine the Larmor frequency, the echo signal mustfirst be phase rotated such that the phase of the echo signal is zero.

Because in general the echo phase is non-zero, the data often needs tobe phase corrected such that the phase is at least close to zero. Thisoperation can be easily performed in the frequency domain. Let X denotethe Fourier transform of a time domain signal x with non-zero phase φ.Let Y denote the Fourier transform of the phase-rotated signal with zerophase. Both X and Y are complex numbers with real and imaginarycomponents. Y can be obtained from X using the following operation:

ReY=Re{X}·cos φ+Im{X}·sin φImY=−Re{X}·sin φ+Im{X}·cos φ  (9)

The phase φ of the time domain signal, x, can be approximated asfollows. First, x is resolved into two orthogonal components S_(r) andS_(x) shown below:

$\begin{matrix}{{S_{r} - {\sum\limits_{n = 1}^{N_{p}}{{x(n)} \cdot {\cos \left( \frac{2\pi \; {f_{T.}\left( {n - 1} \right)}}{f_{s}} \right)}}}}{S_{x} = {\sum\limits_{n = 1}^{N_{p}}{{x(n)} \cdot {\sin \left( \frac{2\pi \; {f_{T.}\left( {n - 1} \right)}}{f_{s}} \right)}}}}} & (10)\end{matrix}$

In the above equation, ƒ_(T) is the transmitter frequency used for theNMR experiment, ƒ_(s) is the sampling frequency, and N_(p) is the numberof sampling points per period, given as:

$\begin{matrix}{N_{p} = \frac{f_{s}}{f_{T}}} & (11)\end{matrix}$

Second, the phase of the echo is determined from the calculated valuesof S_(r) and S_(x) as follows:

$\begin{matrix}{\varphi = {\tan^{- 1}\left\{ \frac{S_{x}}{S_{r}} \right\}}} & (12)\end{matrix}$

FIG. 7 shows the phase-rotated real and imaginary components of theFourier transform of the data shown in FIG. 4. The data are zero paddedto obtain a frequency resolution of 1 KHz. The phase of the echo iscalculated from Eqs. (9)-(12). The real component has a maximum at thecorrect Larmor Frequency of 2.270 MHz, while the imaginary component hasa zero crossing close to the correct Larmor frequency.

The Larmor frequency can also be determined from a DFT of the freeinduction decay (FID) signal of transverse magnetization after theinitial 90° pulse. Similar to CPMG (Carr, Purcell, Meiboom, and Gill)echoes, FID data can also be zero padded and phase rotated for accurateestimation.

FIG. 8 is a flowchart that shows, for a particular embodiment, the stepsfor estimating the Larmor frequency for an NMR tool. In step 200, aninitial estimate of the Larmor frequency is made using, for example, atemperature or Hall probe measurement. FID and/or spin-echo data areobtained using the NMR tool (step 202). If a fast A/D converter isavailable, the data can be directly digitized (step 210). If not, onemay choose to mix the signal. That decision is made in step 204. Ifmixing, then the data signal and mixing signal are mixed (multiplied)(step 206), and the output is filtered to remove the high frequencycomponent and pass the low frequency component (step 208). The lowfrequency component is then digitized (step 210). Several echoes can bestacked (i.e., added together), if desired, to improve thesignal-to-noise ratio.

If the desired frequency resolution requires it, the digitized data canbe zero padded (step 212). Also, if either the real or imaginarycomponents of the DFT are to be used, as opposed to, say, the magnitudeof the DFT, those real or imaginary components should be phase rotatedto zero out the phase. A DFT is then performed on the (zero padded) data(step 214). The modal frequency, or frequency with the maximumamplitude, can be determined from the mode (i.e., peak) of the DFTmagnitude, from the mode of the real component of the DFT, or the zerocrossing of the imaginary component of the DFT (step 216). Determiningthe Larmor frequency now depends on whether the data were digitizeddirectly or mixed and then digitized (step 218). If directly digitized,the Larmor frequency is given by ƒ_(L)=ƒ_(m) (step 220). If the signalwas mixed and then digitized, the Larmor frequency is given byƒ_(L)=ƒ_(r)+ƒ_(m) (step 222). If the solution for the Larmor frequencyhas converged, e.g., the norm of the difference between the currentestimate and the previous estimate is within a given tolerance, then theprocess is halted an the Larmor frequency is determined. Otherwise, theprocess returns to the data acquisition step (step 202) and isiteratively repeated until convergence is achieved (step 224).

Determining the Larmor frequency can be performed regardless of whetherthe data were obtained, for example, using a wireline, sampling, orwhile-drilling NMR tool. Also, though the examples shown and describeduse a discrete Fourier transform, other transform methods could be used,or special cases of a DFT, such as a Fast Fourier Transform, could beused.

An NMR system 300 (FIG. 9) that operates using a Larmor frequencydetermined while the NMR system 300 is disposed in situ includes asource 302 for a static magnetic field, an antenna 304 for transmittingand receiving an electromagnetic signal; electronics 306 to modify theresonance frequency of the antenna 304; a sensor 308 to measure an insitu temperature or an in situ magnitude of the static magnetic field;and a processor 310 capable of: (1) providing an initial estimate of theLarmor frequency using information from the sensor 308, (2) performing aspectral analysis on NMR data acquired by the antenna 304, and (3)controlling the electronics 306.

FIG. 10 illustrates a wellsite system in which one or more aspects ofthe present disclosure may be employed. The wellsite may be onshore oroffshore. In FIG. 10, a borehole 11 is formed in subsurface formationsby rotary drilling in a manner that is well known. Embodiments of thepresent disclosure may also use directional drilling, as will bedescribed hereinafter.

A drill string 12 is suspended within the borehole 11 and has a bottomhole assembly 100 which includes a drill bit 105 at its lower end. Thesurface system includes platform and derrick assembly 10 positioned overthe borehole 11, the assembly 10 including a rotary table 16, kelly 17,hook 18 and rotary swivel 19. The drill string 12 is rotated by therotary table 16, energized by means not shown, which engages the kelly17 at the upper end of the drill string. The drill string 12 issuspended from a hook 18, attached to a traveling block (also notshown), through the kelly 17 and a rotary swivel 19 which permitsrotation of the drill string relative to the hook. As is well known, atop drive system could alternatively be used.

In the example of this embodiment, the surface system further includesdrilling fluid or mud 26 stored in a pit 27 formed at the wellsite. Apump 29 delivers the drilling fluid 26 to the interior of the drillstring 12 via a port in the swivel 19, causing the drilling fluid toflow downwardly through the drill string 12, as indicated by thedirectional arrow 8. The drilling fluid exits the drill string 12 viaports in the drill bit 105, and then circulates upwardly through theannular region between the outside of the drill string 12 and the wallof the borehole 11, as indicated by the directional arrows 9. In thiswell-known manner, the drilling fluid lubricates the drill bit 105 andcarries formation cuttings up to the surface as it is returned to thepit 27 for recirculation.

The bottom hole assembly 100 of the illustrated embodiment includes, butis not limited to, a logging-while-drilling (LWD) module 120, ameasuring-while-drilling (MWD) module 130, a roto-steerable system andmotor 150, and drill bit 105.

The LWD module 120 is housed in a special type of drill collar, as isknown in the art, and can contain one or a plurality of known types oflogging tools. It will also be understood that more than one LWD and/orMWD module can be employed, e.g. as represented at 120A. (References,throughout, to a module at the position of 120 can alternatively mean amodule at the position of 120A as well.) The LWD module includescapabilities for measuring, processing, and storing information, as wellas for communicating with the surface equipment. In the presentembodiment, the LWD module includes a fluid sampling device as well asan NMR device configured according to one or more aspects of the presentdisclosure or otherwise configured to perform at least a portion of amethod within the scope of the present disclosure.

The MWD module 130 is also housed in a special type of drill collar, asis known in the art, and can contain one or more devices for measuringcharacteristics of the drill string and drill bit. The MWD tool furtherincludes an apparatus (not shown) for generating electrical power to thedownhole system. This may typically include a mud turbine generatorpowered by the flow of the drilling fluid, it being understood thatother power and/or battery systems may be employed. In the presentembodiment, the MWD module includes one or more of the following typesof measuring devices: a weight-on-bit measuring device, a torquemeasuring device, a vibration measuring device, a shock measuringdevice, a stick slip measuring device, a direction measuring device, andan inclination measuring device.

FIG. 11 is a simplified diagram of a sampling-while-drilling loggingdevice of a type described in U.S. Pat. No. 7,114,562, incorporatedherein by reference, utilized as the LWD tool 120 or part of an LWD toolsuite 120A. The LWD tool 120 is provided with a probe 6 for establishingfluid communication with the formation and drawing the fluid 21 into thetool, as indicated by the arrows. The probe may be positioned in astabilizer blade 23 of the LWD tool and extended therefrom to engage theborehole wall. The stabilizer blade 23 comprises one or more blades thatare in contact with the borehole wall. Fluid drawn into the downholetool using the probe 26 may be measured to determine, for example,pretest and/or pressure parameters. Additionally, the LWD tool 120 maybe provided with devices, such as sample chambers, for collecting fluidsamples for retrieval at the surface. Backup pistons 81 may also beprovided to assist in applying force to push the drilling tool and/orprobe against the borehole wall.

Referring to FIG. 12, shown is an example wireline tool 900 in whichaspects of the present disclosure may be implemented. The examplewireline tool 900 is suspended in a wellbore 902 from the lower end of amulticonductor cable 904 that is spooled on a winch (not shown) at theEarth's surface. At the surface, the cable 904 is communicativelycoupled to an electronics and processing system 906. The examplewireline tool 900 includes an elongated body 908 that includes aformation tester 914 having a selectively extendable probe assembly 916and a selectively extendable tool anchoring member 918 that are arrangedon opposite sides of the elongated body 908. NMR system 910 is alsoincluded in tool 900. Additional components (not shown) may also beincluded.

One or more aspects of the probe assembly 916 may be substantiallysimilar to those described above in reference to the embodiments shownin FIGS. 10-11. For example, the extendable probe assembly 916 isconfigured to selectively seal off or isolate selected portions of thewall of the wellbore 902 to fluidly couple to the adjacent formation Fand/or to draw fluid samples from the formation F. Accordingly, theextendable probe assembly 916 may be provided with a probe having anembedded plate, as described above. The formation fluid may be expelledthrough a port (not shown) or it may be sent to one or more fluidcollecting chambers 926 and 928. In the illustrated example, theelectronics and processing system 906 and/or a downhole control systemare configured to control the extendable probe assembly 916 and/or thedrawing of a fluid sample from the formation F.

As stated above, the Larmor frequency for an in situ NMR tool can bedetermined and used to acquire NMR data. An NMR tool is provided andplaced in situ, for example, in a wellbore. An initial estimate of theLarmor frequency for the in situ NMR tool is made and NMR data areacquired using the in situ NMR tool. A spectral analysis can beperformed on the NMR data or, optionally, the NMR data are digitized anda transform such as a discrete Fourier transform (DFT) is performed onthe digitized NMR data. The modal frequency of the spectral analysis orDFT is determined, and the Larmor frequency for the in situ NMR tool isdetermined using the modal frequency. The NMR tool is modified totransmit at the determined Larmor frequency and then used to acquirefurther NMR data.

Optionally the NMR data can be mixed with a reference frequency signal.Preferably, the high frequency component from the mixed data is filteredand the low frequency component of the mixed data is digitized, though,if desired, the low frequency component may be filtered and the highfrequency component digitized. Alternatively, the spectrum of theselected component could be analyzed from the analog signal. Regardlessof whether a discrete Fourier transform (DFT) is performed on thedigitized component of the mixed data or another type of spectralanalysis is done, the modal frequency of the spectral analysis isdetermined, and the Larmor frequency is determined for the in situ NMRtool using the modal frequency and the reference frequency.

In view of the above and the figures, it should be readily apparent thatthe present disclosure introduces one or more methods to determine theLarmor frequency for an in situ nuclear magnetic resonance (NMR) tool.Such method may comprise: providing the NMR tool and placing the NMRtool in situ; making an initial estimate of the Larmor frequency for thein situ NMR tool; acquiring NMR data using the in situ NMR tool;performing a spectral analysis on the NMR data; determining the modalfrequency from the spectral analysis; and determining the Larmorfrequency for the in situ NMR tool using the modal frequency. Suchmethod may further comprise modifying the transmission frequency of anantenna on the NMR tool to the determined Larmor frequency, andacquiring further NMR data using the modified NMR tool. The NMR tool maybe a wireline, sampling, or while-drilling tool. The in situ NMR toolmay be disposed in a wellbore during the method. The initial estimate ofthe Larmor frequency may be based on a temperature or Hall probemeasurement. The NMR data may be FID data or spin-echo data. The methodmay further comprise digitizing the NMR data and stacking the digitizedNMR data prior to performing the spectral analysis. The method mayfurther comprise digitizing the NMR data and zero padding the digitizedNMR data prior to performing the spectral analysis. The method mayfurther comprise digitizing the NMR data and rotating the digitized NMRdata to a zero phase angle prior to performing the spectral analysis.Determining the modal frequency may comprise at least one of:determining the mode of the magnitude of the spectral analysis;determining the mode of a phase-rotated real component of the spectralanalysis; and/or determining the zero crossing of a phase-rotatedimaginary component of the spectral analysis. Determining the Larmorfrequency may comprise equating the Larmor frequency to the modalfrequency. The spectral analysis may be one of a discrete Fouriertransform, a fast Fourier transform, or a wavelet transform.

The present disclosure also introduces a method to determine the Larmorfrequency for an in situ nuclear magnetic resonance (NMR) tool,comprising: providing the NMR tool and placing the NMR tool in situ;making an initial estimate of the Larmor frequency for the in situ NMRtool; acquiring NMR data using the in situ NMR tool; mixing the NMR datawith a reference frequency signal; filtering a first frequency componentfrom the mixed data; performing a spectral analysis on a secondfrequency component of the mixed data; determining the modal frequencyof the spectral analysis; and determining the Larmor frequency for thein situ NMR tool using the modal frequency and the reference frequency.Such method may further comprise: modifying the transmission frequencyof an antenna on the NMR tool to the determined Larmor frequency; andacquiring further NMR data using the modified NMR tool. The method mayfurther comprise digitizing the NMR data and zero padding the digitizeddata prior to performing the spectral analysis. The method may furthercomprise digitizing the NMR data and rotating the digitized data to azero phase angle prior to performing the spectral analysis. Determiningthe modal frequency may comprise at least one of: determining the modeof the magnitude of the spectral analysis; determining the mode of aphase-rotated real component of the spectral analysis; and/ordetermining the zero crossing of a phase-rotated imaginary component ofthe spectral analysis. Determining the Larmor frequency may compriseequating the Larmor frequency to the sum of the modal frequency and thereference frequency. The in situ NMR tool may be disposed in a wellboreduring the method. The NMR data may be FID data or spin-echo data. Thespectral analysis may be one of a discrete Fourier transform, a fastFourier transform, or a wavelet transform.

The present disclosure also introduces a nuclear magnetic resonance(NMR) system that operates using a Larmor frequency determined while theNMR system is disposed in situ, comprising: a source for a staticmagnetic field; an antenna for transmitting and receiving anelectromagnetic signal; electronics to modify the resonance frequency ofthe antenna; a sensor to measure an in situ temperature or an in situmagnitude of the static magnetic field; and a processor capable of: (1)providing an initial estimate of the Larmor frequency using informationfrom the sensor, (2) performing a spectral analysis on NMR data acquiredby the antenna, and (3) controlling the electronics.

The present disclosure also introduces a method to determine the Larmorfrequency for an in situ nuclear magnetic resonance (NMR) tool,comprising: providing the NMR tool and placing the NMR tool in situ;making an initial estimate of the Larmor frequency for the in situ NMRtool; acquiring NMR data using the in situ NMR tool; digitizing the NMRdata; performing a discrete Fourier transform (DFT) on the digitized NMRdata; determining the modal frequency of the DFT; and determining theLarmor frequency for the in situ NMR tool using the modal frequency.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

The Abstract at the end of this disclosure is provided to comply with 37C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature ofthe technical disclosure. It is submitted with the understanding that itwill not be used to interpret or limit the scope or meaning of theclaims.

What is claimed is:
 1. A method, comprising: positioning a tool in awellbore; acquiring NMR data using the tool; digitally extracting echodata from the acquired NMR data from the NMR tool; performing a spectralanalysis on the NMR data; and determining a Larmor frequency for the NMRtool.
 2. The method of claim 1, further comprising: modifying atransmission frequency of an antenna on the NMR tool to a determinedLarmor frequency; and acquiring further NMR data using the modified NMRtool.
 3. The method of claim 1, wherein the NMR tool is one of awireline, sampling and while-drilling tool.
 4. The method of claim 1,further comprising: making an initial estimate of a Larmor frequency forthe NMR tool.
 5. The method of claim 1, further comprising: determininga maximum value of the spectral analysis.
 6. The method of claim 5,wherein the determining a Larmor frequency for the NMR tool is donethrough using the maximum value of the spectral analysis.
 7. The methodof claim 4, wherein the initial estimate of the Larmor frequency isbased on one of a temperature and a Hall probe measurement.
 8. Themethod of claim 1, wherein the NMR data are FID data or spin-echo data.9. The method of claim 1, further comprising digitizing the NMR data andstacking the digitized NMR data prior to performing a spectral analysis.10. The method of claim 1, further comprising digitizing the NMR dataand rotating the digitized NMR data to a zero phase angle prior toperforming the spectral analysis.
 11. The method of claim 5, whereindetermining the Larmor frequency comprises at least one of: determininga maximum magnitude of the spectral analysis; determining aphase-rotated real component of the spectral analysis; and determining azero crossing of a phase-rotated imaginary component of the spectralanalysis.
 12. The method of claim 1 wherein determining the Larmorfrequency comprises equating the Larmor frequency to a frequency forwhich the spectrum has a maximum.